Smart and thick-skinned

If you regularly subject your shoes to harsh treatment, it is absolutely essential that they have abrasion-resistant, sturdy outsoles that will not be damaged by sharp-edged stones or abrasive sand.

Till now, solid sole materials were widely touted as the answer to calls for this type of robust footwear, though their weight naturally meant they did not find widespread popularity.

Now the new Bayflex® Integral Plus polyurethane sole material, which the shoe experts at Bayer MaterialScience AG have made further improvements to, offers a winning formula for companies looking to lighten the load for their customers. Soles made from this material, which Bayer is offering as part of the Bayflex® active and Bayflex® fashion product ranges, only have a 'thick skin' where it is most needed – on the underside. That's why they are quite a few grams lighter than standard foams claiming to be light and sturdy.

'The concept behind integral skin polyurethane foam is quite simple,' says Dr. Erhard Michels, who plays a key role in developing tailor-made shoe materials at Bayer MaterialScience AG. 'These flexible materials have a solid, abrasion-resistant outer layer to make the sole incredibly robust and a fine microporous core that not only makes it lighter but also offers excellent shock absorbing properties and first-class resilience. These characteristics are achieved with integral skin polyurethane foam using a simple process. Both the lightweight core and the robust outer skin are formed in a single polyurethane-specific production stage. The desired material parameters, such as the skin zone formation and pore diameter, can be adjusted by creating a certain balance between all the raw materials involved in the foaming process.

'And this is where we were able to put the further developments we have made to the concept into practice,' says Dr. Michels. 'To form the thick, solid outer zone, we not only use our tried-and-tested blowing agent combination technique to create the outer surface we want, we also utilize environmentally friendly new additives. At SIMAC 2004, we will be presenting an additive which ensures that the solid outer skin is only formed where it is needed – on the underside of the sole.'

The Integral Plus additive makes little difference to the normal polyurethane manufacturing process. For example, it can be added in measured doses in paste form to the components. It is also suitable for use in polyester polyurethanes and polyether polyurethanes and can naturally also be utilized for direct soling. The solid outer layer, which can be made up to 2 millimeters thick, increases the sole's resistance against wear and tear. It ensures a well-formed, matt surface and also increases the hardness and rigidity of this lightweight material, which are essential characteristics for today's high-quality, fashionable footwear.

The new process also takes the integral skin foam concept one stage further, by significantly reducing the thickness of the interface on inside the sole where it is not necessary. The Integral Plus process makes a further contribution to reducing the weight of the sole, without detracting from the properties of the integral skin polyurethane foams. And the surface reproduction is extremely good, even with fine soles.

'As a result, we are not only helping footwear manufacturers to meet their customers' needs better, we are also increasing the flexibility of polyurethane technology,' says Dr. Michels. 'We have by no means explored all the possibilities that these versatile synthetic materials offer the footwear market. And when it comes to making the changeover to this innovative process, we are naturally also on hand to offer advice and support.' This is one of the main reasons that the Integral Plus technique has made the successful leap from the laboratory to production at a large German manufacturer's so soon after its development.